1. Field of the Invention
The present invention relates to steel strip continuous annealing furnaces, and more particularly to a steel strip continuous annealing furnace comprising a heating zone, a soaking zone and cooling zones.
2. Description of the Prior Art
Recently, annealing processes for rendering predetermined processability, deep drawing properties and the like to cold-rolled steel strip have been carried out by continuous annealing furnaces. These continuous annealing furnaces each comprise a heating zone for heating the steel strip to a predetermined temperature, a soaking zone for holding the steel strip at a predetermined soaking temperature and a cooling zone for cooling the steel strip to substantially room temperature. The cooling zone further includes a rapidly cooling zone for rapidly cooling the steel strip at a predetermined cooling rate, a controlled cooling zone for controlled cooling the steel strip or holding same at a predetermined temperature to effect overaging treatment, and the like. Consequently, the abovedescribed continuous annealing furnace generally forms a long continuous line, and therefore, it is necessary to render appropriate tensions to the steel strip in the furnace in order to maintain stabilized operating conditions in the furnace.
FIG. 1 is an explanatory view showing the conventional continuous annealing furnace for annealing the black tinplates, silicon steel sheets and the like and the tension of the steel strip in the furnace. In this continuous annealing furnace, the steel strip is annealed through the heating zone 1, soaking zone 2, controlled cooling zone 3 and rapidly cooling zone 4 as the cooling zones, the thereafter, carried out of the furnace. Furthermore, the continuous annealing furnace is provided at the inlet and outlet thereof with bridle rolls 5 for isolating the tension of the steel strip from those in adjacent sections of the steel strip, and the motor torque applied to a dancer roll 6 interposed between the bridle roll 5 at the inlet and the inlet of the heating zone 1 is changed, so that the tension of the steel strip in the furnace can be varied.
However, since the steel strip is progressively heated to be thermally expanded in the heating zone 1, there is a possibility that the steel strip is loosened to move in a zigzag fashion, if the tension of the steel strip is low. Furthermore, at the outlet of the heating zone 1 and in the soaking zone 2, the steel strip is heated to a high temperature to be softened and becomes low in its yielding point, and hence, if the tension of the steel strip is too high, then there occurs a possibility of that the value of plastic deformation of the steel strip is increased, thus resulting elongation in the longitudinal direction and in shrinkage in the widthwise direction. Namely, it is desirable that the tension of the steel strip in the heating zone 1 is set at a high value and the tensions of the steel strip at the outlet of the heating zone 1 and in the soaking zone 2 are set at values lower than the former.
However, in the conventional continuous annealing furnace the bridle rolls 5 are provided only at the inlet and outlet of the furnace as described above, the tension of the steel strip over all the furnace is held at a predetermined value by the dancer roll 6 provided at the inlet of the furnace, whereby the actual tension of the steel strip is in a uniformly stretched condition over all the furnace as shown in a chart of FIG. 1. Consequently, in the continuous annealing furnace as described above, it is impossible to obtain such a discontinuous variation of the tension of the steel strip in the furnace that the tension of the steel strip in the heating zone 1 is set at the high value and the tensions of the steel strip at the outlet of the heating zone 1 and in the soaking zone 2 are set at the values lower than the former.
FIG. 2 is an explanatory view showing the conventional annealing furnace for annealing the soft black tinplates or the cold-rolled steel sheets for drawing and the tension of the steel strip in the furnace. In this continuous annealing furnace, the steel strip is annealed through a heating zone 11, a soaking zone 12 and a quenching zone 13, a controlled cooling zone 14 and a rapidly cooling zone 15 as the cooling zones, and thereafter, carried out of the furnace. The tension control in this continuous annealing furnace, similarly to that in the continuous annealing furnace as shown in FIG. 1, is effected in such a manner that the tension of the steel strip is isolated by the bridle rolls 5 provided at the inlet and outlet of the furnace and the motor torque of the dancer roll 6 interposed between the bridle roll 5 at the inlet and the heating zone 11 is changed.
However, the steel strip is rapidly cooled at a high cooling rate in the quenching zone 13, whereby irregularities in cooling tend to occur, and if the tension is excessively high, then irregular shapes of the steel strip and cooling buckling may take place. Consequently, the tension of the steel strip in the quenching zone 13 is required to be set at a value lower than those in other zones.
Nevertheless, in the conventional continuous annealing furnace, only the tension control over all the furnace is effected as described above, whereby the actual tension of the steel strip is in a uniformly stretched condition over all the furnace as shown in a chart of FIG. 2, and hence, the tension of the steel strip only in the quenching zone 13 cannot be set at a discontinuously low value.
More specifically, in the conventional continuous annealing furnace, the conditions of tension in the respective zones of the furnace cannot be controlled independently of one another, thus presenting the problems including the ruptures in the steel strip due to the movement of the steel strip in a zigzag fashion, damages caused to the furnace, fluctuations in width of the steel strip, irregular shapes of the steel strip, local fluctuations in shapes of the steel strip and the like. In addition, in some of the conventional continuous annealing furnaces, the motor torque for the hearth rolls are controlled in the respective zones, so that the tensions of the steel strip in the respective zones can be varied. However, the motor for the hearth rolls is adapted to have a so low capacity as to supplement the torque required for the rotation of the rolls, so that such a control cannot be effected as to bring about great changes in tension between the respective zones.